The hair cosmetic industry offers a great diversity of products targeting the beautification of hair. Care products have become part of our everyday life. Besides treatments at home which commonly comprise shampooing and styling, coloration of hair and also reshaping have become the most important treatments in hairdresser's salons. In European countries the focus has been on permanent waving, whereas in American countries, mainly in Latin American countries, people often have very curly or frizzy hair which is hardly manageable hair straighteners stand out with a high demand by customers aiming at beauty, social acceptance and ease of daily hair maintenance.
The market has therefore offered various products for reshaping hair, either from straight hair into curly or wavy hair, or from frizzy or curly hair into straight hair which is also known as smoothing of hair.
One of the most important problems is that, although active ingredients are able to straighten wavy, curly or frizzy hair, said substances are often impaired by unfavorable safety profiles and high aggressiveness for hair.
Over the years three methods have been established for reshaping hair.
In the first two cases described below, a significant percentage of cystine disulfide bonds have to be disrupted. While there are other types of bonds between the polypeptides in hair fibers, such as ionic bonds, the permanent shape of the hair is essentially dependent on the disulfide bonds of cystine units. The chemical disruption of disulfide bonds is generally combined with mechanical straightening of the hair, such as combing. Straightening generally occurs due to changes in the relative positions of neighboring polypeptide chains within the hair fiber.
Method 1: Use of Reducing Agents
One method for chemical straightening of hair is substantially identical with the technique used in permanent waving. Relaxing is based on the chemical reduction of the cystine structure into two cysteine units followed by reconfiguration of the hair and restoring of cystine structures.
In general, the sulfur to sulfur cystine bonds in human hair maintain the hair in a naturally straight or curly configuration. Polypeptide chains are crosslinked by sulfur to sulfur bonds in cystine units. In order to permanently reshape the hair into a different configuration, a significant amount of the disulfide bonds (—S—S—) must be broken to form two (—SH) groups. Protein chains of the hair are locally disconnected from each other and the hair can now be reshaped. After the hair is reconfigured in a desired position, new disulfide bonds are reestablished, e.g. by the application of a hydrogen peroxide composition. The formation of the newly formed linking (—S—S—) groups leads to a longer lasting straightening effect. The commonly used active ingredient which is used for breaking the disulfide bonds is ammonium thioglycolate.
In more ancient times hydrogensulfite solutions and/or sulfite solutions and/or disulfite solutions were used to cleave disulfide bridges into a thiol group and a Bunte salt moiety; this method was found by Clark and Speakman in 1932 (see: W. Umbach: Kosmetik—Entwicklung, Herstellung and Anwendung kosmetischer Mittel, 2nd edition, Georg Thieme Verlag, Stuttgart, 1995).
Reducing compositions which contain higher amounts of hydrogensulfite, sulfite or disulfite do not have the strong intrinsic odor of compositions which comprise mercaptans. Fixing is carried out with the help of heat or by an oxidizing agent with the formation of new disulfide bridges, whereby the Bunte salt moieties are not available for fixing in the course of the treatment for reshaping. However, the degree of hair damage was high and the hair quality was considerably suffering. For these reasons and also for safety reasons this method was no longer applied.
One of the disadvantages of reducing agents is in the subsequent use of an oxidizing neutralizer, such as hydrogen peroxide, to chemically relink the hair keratin disulfide bonds and also deactivate the reducing agent. As reducing compounds are usually buffered in an alkaline state, any excess of hydrogen peroxide must be removed at the same time to avoid stronger lightening or discoloration of the hair. Discoloration is often observed if dyed hair is treated with compositions containing thioglycolate.
The inconveniences of the use of reducing agents which are able to break-off the disulfide bonds is the pungent odor of the thio compounds, mainly the strong smell of ammonium thioglycolate.
Method 2: Use of Alkali Agents
For many years solutions of alkali hydroxides have been used which efficiently straighten hair. Sodium hydroxide is commonly used in chemical relaxers and provides long lasting effects. Alkali metal hydroxides are key ingredients in products that are referred to as “lye” relaxers.
In U.S. Pat. No. 4,304,244 guanidine hydroxide was claimed as another common option of relaxer chemical; relaxers containing this compound are also referred to as “no-lye” relaxers. However, the strength of both relaxer types varies from a pH of 12 to 14.
Hair treatments with a formulation containing hydroxide based relaxers include two consecutive chemical steps. In the first step the treatment leads to the cleavage of cystine (—S—S—) bridges whereby a cysteine moiety and a dehydroalanine moiety are formed. According to C. Zviak, The Science of Hair Care, pp. 185-186 (1986) one sulfur atom is removed from the hair and converted into a HOS− ion. In this situation important parts of the polypeptide chains are not crosslinked and the hair can be reshaped. In the second step the cysteine thiol group reacts with the double bond of the dehydroalanine moiety to form a lanthionine bridge (—CH2—S—CH2—) which largely restores mechanical stability of the hair.
Treatments with hydroxides, in particular with alkali hydroxides, are very effective in producing stable, but lead to completely irreversible, crosslinks in the treated hair so that subsequent reshaping of the hair is excluded. In addition, the applied chemical compositions are very harsh to both the scalp and the hair and the use of such compositions has resulted in numerous instances of scalp irritation and burning, in substantial reduction of the strength of treated hair and in some cases to considerable hair loss as disclosed in Int. J. of Cosmetic Science, 2014, (36), 2-11.
Prolonged exposure of hair to such a strong alkali can even dissolve the hair. Various attempts have therefore been made to replace the strong alkali metal hydroxides by less active hydroxides. However, these compositions are not satisfactory, neither in terms of relaxing nor in cosmetic terms.
The aforementioned straightening systems which are all based on breaking off the cystine bridges have a deteriorating effect on the hair.
Another approach for reshaping hair is the introduction of crosslinks caused by aldehydes.
Method 3: Use of Formaldehyde
U.S. Pat. No. 2,390,073 claimed a hair treatment system based on formaldehyde or a formaldehyde releasing compound. According to said document, formaldehyde should establish new crosslinks in the polypeptide structures which overcompensate the forces of the natural cystine bonds and provide the straightening effect. Typical concentrations of formaldehyde in such straighteners were between 5 and 10 percent. Later-on formaldehyde adducts have been commercialized in various straightener compositions until the use of formaldehyde was restricted to concentrations of 0.2% maximum for safety and regulation observance. Considerable amounts of formaldehyde based straightener compositions present at 5 to 10% by weight evaporate in the course of the hair treatment since the processes require blow dryer or hot flat iron treatments that causes the product to steam. In consideration of the toxicological profile of formaldehyde and its adducts it was finally concluded that there is no safe level of formaldehyde assessment for this product.
US20130118520 discloses that human hair can also be straightened by the use of higher amounts of glyoxylic acid at increased temperatures, e.g. by means of a shaping iron.
However, the use of glyoxylic acid at higher temperatures frequently leads to lightening of hair, in particular of artificially colored hair, which gave reason for dissatisfaction. The use of glyoxylic acid as substitute of formaldehyde in semi-permanent hair straightening has been thoroughly discussed in Int. J. of Cosmetic Science, 2014, (36), 459-470.
It is evident that straightening or relaxing the curls of very curly hair improves the manageability and ease of styling such hair, independently of the applied techniques and compositions described above. Currently there is a strongly increasing demand for the hair care products referred to as “smoothing agents”, “straighteners” or “hair relaxers,” which can reshape naturally curly hair.
Various techniques for waving or straightening hair have been applied which are based on chemical methods, physical methods and also a combination of both.
In particular if glyoxylic acid is used for straightening hair where lightening occurs it is customary to perform a two-step process where the hair is smoothed in the first step, then tinted with a color in another step to recover the previous hair color. However, two subsequent hair treatments are hair damaging and laborious.
DE19859722 discloses colorants, mainly oxidative colorants, comprising lower amounts of oxocarboxylic acids and derivatives thereof which should intensify the hair color under standard processing conditions. The disclosed compositions are not suitable for straightening and coloring hair in one step. Moreover, only oxidative dyes may be used.
EP 1326579 uses short chained carboxylic acids for stabilizing color results.
EP 2258337 discloses cleansing and conditioning compositions comprising direct dyes and glyoxylic acid in concentrations insufficient to provide semipermanent straightening effect. US 2002/192175 discloses straightening compositions comprising alkaline agents and cationic dyes causing an irreversible straightening since the alkaline agents break down the disulfide bridges of the keratin fibers with consequent breaking of the hair and irritation of the scalp.